Can a bank change the rates/fees between pre-approval/unconditional approval and signing?

Hope this is a better sub for my question, as I tried asking in r/AusFinance and it got removed (not sure why, but it's OK I probably didn't realise I broke one of their rules)

We are in a bit of a bind and feel honestly tricked by our bank. So we bought a new PPOR, and had to remortgage our existing loan as part of the conditions for the new loan. That's fine.

Remortgage comes through, have pre-approval for the second loan, made offer on place we liked, offer accepted. Loan was unconditionally approved at a rate of 6.29% and $6k in LMI. These were the figures or mortgage broker had the whole time, in every signed loan choice agreement with him. So we paid the deposit to the vendor as we thought finance all sorted. This all happened on Tuesday. We're happy as and excited.

Then yesterday(thursday) my broker called us, apparently the bank (Great Southern Bank) 'made a mistake' on their end. Our loan docs are ready to sign, but the rate is now 6.59% and the LMI $9K. Or repayments would now be an extra $100 a month. Still doable, but honestly not the point.

I feel tricked honestly. We don't have time to go through another lender as we would have to remortgage the apartment too and the deposit is paid, we have settlement booked in for the new place and don't want to lose it out fuck over the vendor. Or broker is trying to push back at least on the rate but I just... I mean I don't get how they can claim 'oopsie' on the rate and LMI when we already have those in writing in several earlier versions of our loan choice agreement? We chose them for the lowest rate and LMI and now they are the highest of the other options but due to the timing of everything we are stuck with them.

So basically, can banks just do this? Claim they made a mistake at the last minute when they have you over the barrel like this? Do we have any other recourse or just have too deal with it now and remortgage down the line again?